Polarized 3D glasses
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Polarized 3D glasses create the illusion of three-dimensional images by restricting the light that reaches each eye, an example of stereoscopy. To present a stereoscopic motion picture, two images are projected superimposed onto the same screen through orthogonal polarizing filters. The viewer wears low-cost eyeglasses which also contain a pair of orthogonal polarizing filters. As each filter only passes light which is similarly polarized and blocks the orthogonally polarized light, each eye only sees one of the images, and the effect is achieved.
The difficulty arises because light reflected from a motion picture screen tends to lose a bit of its polarization. However, this problem is eliminated if a 'silver' or Aluminized screen is used. This means that a pair of aligned DLP projectors, some polarizing filters, a silver screen, and a computer with a dual-head graphics card can be used to form a relatively low-cost (under US$10 000 in 2003) system for displaying stereoscopic 3d data simultaneously to tens of people wearing polarized glasses. Such a system, called a GeoWall, has been used for several years now in the Earth Sciences thanks to the GeoWall Consortium, with several open source and commercial packages available.
When stereo images are to be presented to a single user, it is practical to construct an image combiner, using partially silvered mirrors and two image screens at right angles to one another. One image is seen directly through the angled mirror whilst the other is seen as a reflection. Polarized filters are attached to the image screens and appropriately angled filters are worn as glasses. A similar technique uses a single screen with an inverted upper image, viewed in a horizontal partial reflector, with an upright image presented below the reflector, again with appropriate polarizers. Polarizing techniques are most simply used with cathode ray technology, as polarizers are used within ordinary LCD screens for control of pixel presentation - this can interfere with these techniques.
In 2003 Keigo Iizuka discovered an inexpensive implementation of this principle on laptop computer displays using cellophane sheets [1].
Polarized stereoscopic pictures have been around since 1936, when Edwin H. Land first applied it to motion pictures. The so called "3-D movie craze" in the years 1952 through 1955 was almost entirely offered in theaters using polarizing projection and glasses. Only a minute amount of the total 3D films shown in the period used the anaglyph color filter method. What is new is the use of digital projection, and also the use of sophisticated IMAX 70mm film projectors, with very reliable mechanisms. A whole new generation of 3D animation films are beginning to show up in the theaters, all using some form of polarization. Polarization is not easily applied to home 3-D broadcast or DVD presentation. At this point only anaglyph glasses may be used to view the new HD shows and are beginning to be aired occasionally by NBC and the Discovery Channel.
In optometry, polarized glasses are used for various tests of binocular depth perception (i.e. stereopsis).
